Device purpose:

Pressing the pushbutton of the transmitter, a sound and/or light alert is activated in the receiver. The system uses no wiring or radio frequencies: the transmitted signal is conveyed into the mains supply line. It can be used at home, in any room from attic to cellar, simply plugging transmitter and receiver in the wall mains sockets. Transmission range can be very good, provided both units are connected to the mains supply within the control of the same light-meter.

Transmitter circuit operation:

Q1 and Q2 are wired as a Darlington pair to obtain the highest possible output from a Hartley type oscillator running at about 135KHz frequency. The 230Vac mains is reduced to 30Vdc without the use of a transformer by means of C2 reactance, a two diode rectifier cell D1 & D2 and Zener diode D3.
The oscillator output is taken from L1 secondary winding and injected into the mains wiring by means of C1.

Receiver circuit operation:

The 135KHz sinewave generated by the transmitter is picked-up from mains wiring by C1 and selected by the tuned circuit L1-C3. Q1 greatly amplifies the incoming sinewave and converts it in a 12V-peak squarewave. D4 & D5 limit the input voltage at Q1 base to less than 1V-peak to avoid damaging of the transistor due to the high voltage transients frequently occurring on the mains line. D6 eliminates any negative component of the signal and Q2 drives the load. C7 is necessary to smooth the signal residues appearing across the load.
The 12Vdc supply for this unit is obtained as described above for the transmitter circuit.

Notes:

Transmitter and receiver coils L1s must be tuned regulating their ferrite cores to obtain maximum output at C3 leads, either in transmitter and receiver.

This setup is better done using an oscilloscope and placing the two units as far as possible to each other.

The tuning of the coils at 135KHz frequency should be obtained with the ferrite core almost totally inserted in its slot, if 455KHz IF transformers are used for both L1s.

Using IF transformers different from those specified, a change in both C3s value could be needed. The value of these capacitors may vary from 1 to 3.3nF but must be the same in transmitter and receiver.

The load can be a beeper, a LED or both. Omitting the beeper and choosing the LED as the only load, its limiting resistor R7 should be reduced in value to about 1K, to increase device brightness. In this case, a 10mm. diameter LED type or greater, can also be useful .

Warning! These units are connected to 230Vac mains, then some parts in the circuit boards are subjected to lethal potential! Avoid touching the circuits when plugged and enclose them in plastic boxes.